1. Field of the Invention
The present invention relates to a heterogeneous cellular communication system. More particularly, the present invention relates to a resource allocation method and apparatus for mitigating inter-cell interference in the heterogeneous cellular communication system.
2. Description of the Related Art
In order to improve performance of and to meet user requirements for a high speed data communication service, recent wireless communication systems have to secure a better radio environment. For example, a radio environment may be improved by removing an indoor shadow area, and utilizing limited frequency resources efficiently. Recently, there has been research on the use of a femto cell (e.g., a femto base station) for facilitating high speed data transmission service.
A femto cell is an indoor base station operating at low power and having relatively small cell size. For example, a femto cell may be approximately 10˜50 m. Due to the femto cell's short coverage, it is advantageous to use the femto cell to improve the indoor communication service quality. However, deployment of a plurality of femto cells in a macro cell can cause several technical problems due to differences in the cell structures of femto cells and macro cells. One of the significant problems is inter-cell interference between heterogeneous cells (i.e. interference between the macro cell and the femto cell).
One of the simplest methods to address the inter-cell interference problem between heterogeneous cells is to separate the given frequency resource into dedicated channels. In this method, a part of the entire frequency resource is reserved for the macro cell, and the remaining frequency resource is allocated to femto cells. This method is advantageous for mitigating interference because of its simplicity. However, because a specific frequency resource can be used for only one of a macro cell and a femto cell, resource utilization efficiency is reduced. In order to address the low resource utilization efficiency problem of this method, research is being conducted on a technique that enables a macro cell and a femto cell to use the same frequency resource (e.g., a co-channel) while mitigating inter-cell interference.
There are many techniques known to mitigate inter-cell interference. One technique that is known to mitigate inter-cell interference is a modified frequency reuse method. The modified frequency reuse method adopts a macro cell structure with Fractional Frequency Reuse (FFR). In this method, the femto cell is assigned, according to its location, an orthogonal Frequency Assignment (FA) resource which is not used by the macro cell, thereby mitigating inter-cell interference between heterogeneous cells. However, because the available FA resource is fixed according to the location of the femto cell, the frequency reuse rate of the femto cell is very low.
Another related approach to mitigate inter-cell interference is to allocate a sub-channel causing the least interference with the femto user terminal (e.g., the femto terminal) based on the downlink interference information provided by the macro cell. However, this method is designed to maximize the performance of the femto user terminal without consideration of the interference of the femto cell to an adjacent macro user terminal. Accordingly, if a macro user terminal is located in an indoor region or a macro cell boundary region at which the macrocell signal is weak, then the adjacent femtocell may not be able to detect the signal from the macrocell and, as a consequence, the femto cell may erroneously determine (e.g., mis-detect) that the sub-channel preoccupied by the macro cell is not being used. If the femtocell uses the sub-channel preoccupied by the macro cell, then the sub-channel signal of the femto cell may significantly interfere with the adjacent macro user. Further, if the transmission power of signals transmitted across the sub-channel in the femto cell are increased to maximize the femto cell's performance, then the interference caused by the sub-channel signal of the femto cell may be increased. For example, the interference may extend to the neighbor femto user terminals as well as neighbor macro user terminals.
Therefore, a need exists for a method and apparatus for adjusting the transmission power below an appropriate level in accordance with the channel environment.
The above information is presented as background information only to assist with an understanding of the present disclosure. No determination has been made, and no assertion is made, as to whether any of the above might be applicable as prior art with regard to the present invention.